Reactions of pyrazole with unsaturated triangular clusters of rhenium. Solid-state and solution characterization of an intramolecular N-H?π hydrogen bond

Article abstract of DOI:10.1021/om9800849

The reaction of the unsaturated anion [Re₃(μ-H)₄(CO)₁₀]^- with pyrazole (used as solvent at 80°C) proceeds in a stepwise manner, at first giving the [Re₃(μ-H)₃(μ-η²-pz)(CO) ₁₀]^- anion (2, pz = pyrazolate), with H₂ evolution. Spectroscopic characterization has shown that 2 contains a pz ligand bridging through the two N atoms, one edge of the triangular cluster. At longer times, CO is evolved and 2 transforms into the novel species [Re₃(μ-H)₃(μ-η²-pz)-(CO) ₉(Hpz)]^- (3), containing one pz ligand bridging a cluster edge and a second pyrazole molecule terminally bonded on the third vertex of the triangle, replacing an axial carbonyl of 2. The treatment with CO (atmospheric pressure, room temperature) slowly restores 2. A single-crystal X-ray analysis of 3 has revealed the syn coordination of the two heterocycles with respect to the metal triangle plane and an intramolecular hydrogen bond between the N-H proton of the pyrazole ligand and the aromatic π-electrons of the bridging pyrazolate. There is NMR evidence that this intramolecular hydrogen bond is maintained in CD₂Cl₂ solution: (i) the N-H protonic resonance is significantly shifted upfield, with respect to the values typical of complexes containing Hpz, due to the screening effect of the π-electron cloud; (ii) a 2D NOESY experiment has shown a dipolar interaction between H_α of the pyrazolate and the N-H protonic resonance (and, to a lesser extent, H_α of the pyrazole). In acetone, intermolecular N-H?solvent interactions replace, in part, the intramolecular H bond and different conformers are populated in different ratios on varying the temperature, as shown by variable-temperature and nOe (ID and 2D) experiments. Further reacting 1 in molten pyrazole causes fragmentation of the triangular cluster moiety with formation of the novel dinuclear anion [Re₂(μ-H)(μ-η²-pz)₂(CO) ₆]^- (4) and of the neutral complex [Re(pz)-(CO)₃(Hpz)₂]. A single-crystal X-ray analysis of the [NEt₄]⁺ salt of 4 showed that the dinuclear anion contains two fac-Re(CO)₃ units linked by one hydrido and two pyrazolato bridging ligands, with the two μ-η²-pz units almost orthogonal to each other. A more selective route to anion 3 is provided by the reaction of the 46 valence electron complex [Re₃(μ-H)₄-(CO)₉(Hpz)]^- (6) with Hpz. On using pyrazole deuterated in the nitrogen site, HD evolution has been demonstrated. The treatment of 6 with MeCN affords, even if very slowly, another derivative containing a bridging pyrazolate, namely the anion [Re₃(μ-H)₃(μ-η²-pz)(CO) ₉-(NCMe)]^-, characterized spectroscopically. Also, the unsaturated anion [Re₃(μ-H)₄(CO)₉(PMe₂-Ph)] ^- reacts with Hpz, giving two isomers of the [Re₃(μ-H)₃(μ-η²-pz)(CO) ₉(PMe₂Ph)]^- anion (with the phosphine in an axial and equatorial position, respectively). The reactions rates of the 46 valence electron complexes [Re₃(μ-H)₄(CO)₉L]^- with pyrazole increase on increasing the electron density on the cluster (in the order L = Hpz > PMe₂Ph > CO), suggesting that the rate is controlled by the interaction of a hydridic H ligand with the acidic N-H bond of pyrazole. In no case has activation of the ortho C-H bond of pyrazole been observed.

Full text of DOI:10.1021/om9800849

3282
Organometallics 1998, 17, 3282-3292
Rea ction s of P yr a zole w ith Un sa tu r a ted Tr ia n gu la r
Clu ster s of Rh en iu m . Solid -Sta te a n d Solu tion
Ch a r a cter iza tion of a n In tr a m olecu la r N-H‚‚‚π
Hyd r ogen Bon d
T. Beringhelli,* G. D’Alfonso,* M. Panigati, and F. Porta
Dipartimento di Chimica Inorganica, Metallorganica e Analitica, e Centro CNR CSMTBO,
Via Venezian 21, 20133 Milano, Italy
P. Mercandelli, M. Moret,* and A. Sironi
Dipartimento di Chimica Strutturale e Stereochimica Inorganica, e Centro CNR CSMTBO,
Via Venezian 21, 20133 Milano, Italy
Received February 9, 1998
The reaction of the unsaturated anion [Re3(µ-H)4(CO)10]- with pyrazole (used as solvent
at 80 °C) proceeds in a stepwise manner, at first giving the [Re3(µ-H)3(µ-η2-pz)(CO)10]- anion
(2, pz ) pyrazolate), with H2 evolution. Spectroscopic characterization has shown that 2
contains a pz ligand bridging through the two N atoms, one edge of the triangular cluster.
At longer times, CO is evolved and 2 transforms into the novel species [Re3(µ-H)3(µ-η2-pz)-
(CO)9(Hpz)]- (3), containing one pz ligand bridging a cluster edge and a second pyrazole
molecule terminally bonded on the third vertex of the triangle, replacing an axial carbonyl
of 2. The treatment with CO (atmospheric pressure, room temperature) slowly restores 2.
A single-crystal X-ray analysis of 3 has revealed the syn coordination of the two heterocycles
with respect to the metal triangle plane and an intramolecular hydrogen bond between the
N-H proton of the pyrazole ligand and the aromatic π-electrons of the bridging pyrazolate.
There is NMR evidence that this intramolecular hydrogen bond is maintained in CD2Cl2
solution: (i) the N-H protonic resonance is significantly shifted upfield, with respect to the
values typical of complexes containing Hpz, due to the screening effect of the π-electron
cloud; (ii) a 2D NOESY experiment has shown a dipolar interaction between HR of the
pyrazolate and the N-H protonic resonance (and, to a lesser extent, HR of the pyrazole). In
acetone, intermolecular N-H‚‚‚solvent interactions replace, in part, the intramolecular H
bond and different conformers are populated in different ratios on varying the temperature,
as shown by variable-temperature and nOe (1D and 2D) experiments. Further reacting 1
in molten pyrazole causes fragmentation of the triangular cluster moiety with formation of
the novel dinuclear anion [Re2(µ-H)(µ-η2-pz)2(CO)6]- (4) and of the neutral complex [Re(pz)-
(CO)3(Hpz)2]. A single-crystal X-ray analysis of the [NEt4]+ salt of 4 showed that the
dinuclear anion contains two fac-Re(CO)3 units linked by one hydrido and two pyrazolato
bridging ligands, with the two µ-η2-pz units almost orthogonal to each other. A more selective
route to anion 3 is provided by the reaction of the 46 valence electron complex [Re3(µ-H)4-
(CO)9(Hpz)]- (6) with Hpz. On using pyrazole deuterated in the nitrogen site, HD evolution
has been demonstrated. The treatment of 6 with MeCN affords, even if very slowly, another
derivative containing a bridging pyrazolate, namely the anion [Re3(µ-H)3(µ-η2-pz)(CO)9-
(NCMe)]-, characterized spectroscopically. Also, the unsaturated anion [Re3(µ-H)4(CO)9(PMe2-
Ph)]- reacts with Hpz, giving two isomers of the [Re3(µ-H)3(µ-η2-pz)(CO)9(PMe2Ph)]- anion
(with the phosphine in an axial and equatorial position, respectively). The reactions rates
of the 46 valence electron complexes [Re3(µ-H)4(CO)9L]- with pyrazole increase on increasing
the electron density on the cluster (in the order L ) Hpz > PMe2Ph > CO), suggesting that
the rate is controlled by the interaction of a “hydridic” H ligand with the acidic N-H bond
of pyrazole. In no case has activation of the ortho C-H bond of pyrazole been observed.
In tr od u ction
change processes between the hydrides and the ortho
position of the pyridine revealed the reversibility of the
C-H oxidative addition. Moreover, under high H2
pressure it was possible to expel the pyridine ligand
It has been previously shown that the unsaturated
triangular cluster anion [Re3(µ-H)4(CO)10]- (1, 46 va-
lence electrons) is able to activate the ortho C-H bond
of pyridine, giving a derivative containing an ortho-
metalated pyridine moiety.1 Regioselective H/D ex-
(1) Beringhelli, T.; Ciani, G.; D’Alfonso, G.; Proserpio, D. M.; Sironi,
A. Organometallics 1993, 12, 4863.
S0276-7333(98)00084-3 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 06/27/1998

Reactions of Pyrazole with Clusters of Re
Organometallics, Vol. 17, No. 15, 1998 3283
Sch em e 1
Sch em e 2
from the cluster, restoring the unsaturated starting
anion 1.2 On using deuterated pyridine, evidence was
obtained supporting the mechanism shown in Scheme
1, involving pyridine coordination and subsequent re-
ductive elimination of two H ligands, affording the
coordinatively unsaturated species I2. This key inter-
mediate was also formed by the C-H reductive elimina-
tion in the reverse step; therefore, the competition
between intramolecular C-H oxidative addition and
coordination of a different ligand, such as CO, could be
studied starting either from the unsaturated anion 1
or from the saturated orthometalated derivative [Re3-
(µ-H)3(µ-η2-NC5H4)(CO)10]-.2
Due to continuing interest in the reactivity of aro-
matic nitrogen heterocycles with transition metals, in
particular for the activation of C-H and N-H bonds in
these molecules,3 we decided to investigate the reactions
of 1 with a different heterocycle, such as pyrazole.
Despite being a poor base,4 pyrazole is a good σ-donor,
acting as a monodentate ligand through its pyridinic
nitrogen atom, or as a bidentate bridging ligand in the
form of the pyrazolato anion.5,6 In our system, the
coordination of pyrazole would give an intermediate
similar to I1 of Scheme 1, offering the chance to study
the competition between C-H or N-H activation on the
Re cluster. In the related reaction of pyrazole with the
Os3(CO)10 fragment, it has been previously shown3c,7
that the products of activation of the N-H and CR-H
bonds are formed, with the first one dominating.
Resu lts a n d Discu ssion
Step w ise Rea ction of 1 in Molten P yr a zole. The
reactivity of 1 with pyrazole has been studied using the
ligand itself as solvent, carrying out the reactions at a
temperature just above the melting point of pyrazole
(70 °C). In these conditions, mixtures of the four species
[Re3(µ-H)3(µ-η2-pz)(CO)10]- (2), [Re3(µ-H)3(µ-η2-pz)(CO)9-
(Hpz)]- (3), [Re2(µ-H)(µ-η2-pz)2(CO)6]- (4), and [Re(pz)-
(CO)3(Hpz)2] (5) described below are formed.
Attempts to improve the selectivity on performing the
reaction in THF, at lower temperatures, were unsuc-
cessful. At room temperature, no reaction was observed
after 24 h upon addition of 50 equiv of pyrazole to a
THF solution of 1. On increasing both temperature (40
°C) and pyrazole (700 equiv), complete disappearance
of 1 occurred after 24 h but spectroscopic monitoring
showed the formation of a mixture containing all of the
species 2-5. No improvement of selectivity was ob-
tained on lowering the amount of pyrazole.
(2) Beringhelli, T.; Carlucci, L.; D’Alfonso, G.; Ciani, G.; Proserpio,
D. M. J . Organomet. Chem., 1995, 504, 15.
However, the selectivity of the reaction in molten
pyrazole could be partially controlled by a careful choice
of the reaction time. When the reaction was stopped
after a very short time (2 min after the complete
dissolution of [NEt4]1), the main reaction product
(>90%) was spectroscopically identified as the [Re3(µ-
H)3(µ-η2-pz)(CO)10]- anion (2, pz ) pyrazolate) of Scheme
2. The presence of the pyrazolate ligand bridging one
edge of the triangular cluster is indicated by the two
resonances, in a 1:2 ratio, in the region of the aromatic
protons. The Cs symmetry imposed to the anion by the
N,N′-exo-bidentate coordination of pz is confirmed by
the presence of two hydridic resonances, in a 1:2 ratio,
down to 193 K. The δ values of the hydridic signals
(-11.56 and -13.59 ppm, 1:2, acetone-d6) show the
(3) See for instance: (a) Garcia, E.; Kolwaite, D. S.; Rosenberg, E.;
Hardcastle, K.; Ciurash, J .; Duque, R.; Gobetto, R.; Milone, L.; Osella,
D.; Botta, M.; Dastru`, W.; Viale, A.; Fiedler, I. Organometallics 1988,
17, 415. (b) Kabir, S. E.; Kolwaite, D. S.; Rosenberg, E.; Hardcastle,
K.; Cresswell, W.; Grindstaff, J . Organometallics 1995, 14, 3611. (c)
Agarwala, R.; Azam, K. A.; Dilshad, R.; Kabir, S. E.; Miah, R.;
Shahiduzzaman, M.; Hardcastle, K. I.; Rosenberg, E.; Hursthouse, M.
B.; Malik, K. M. A. J . Organomet. Chem. 1995, 492, 135 and references
therein.
(4) (a) Reedijk, J. In Comprehensive Coordination Chemistry; Wilkin-
son, G., Ed., Pergamon Press: New York, 1987; Vol. 2, p 73. (b)
Catalan, J .; Abboud, J . L. M.; Elguero, J . Adv. Heterocycl. Chem. 1987,
41, 187.
(5) La Monica, G.; Ardizzoia, G. A. Prog. Inorg. Chem. 1997, 46, 151
and references therein.
(6) Sadimenko, A. P.; Basson, S. S. Coord. Chem. Rev. 1996, 147,
247 and references therein.
(7) Shapley, J . R.; Samkoff, D. E.; Bueno, C.; Churchill, M. R. Inorg.
Chem. 1982, 21, 634.

3284 Organometallics, Vol. 17, No. 15, 1998
Beringhelli et al.
Ta ble 1. Va r ia ble-Tem p er a tu r e 1H HMR Da ta for
th e P yr a zole Liga n d a in [NEt4]3a (ca . 0.035 M)b
CD2Cl2
HR Hγ
acetone-d6
HR Hγ
T (K) N-H
Hâ
N-H
Hâ
193
213
233
253
273
298
7.12
6.89 7.68 6.13 12.19 7.64 6.61 6.07
11.71 7.54 6.84 6.05
11.30 7.47 6.98 6.04
10.95 7.41 7.08 6.04
10.61 7.35 7.17 6.04
7.78
6.91 7.68 6.11 10.25 7.30 7.25 6.04
a
The bridging pyrazolate gives resonances at 7.31 (HR,γ), 6.03
(Hâ) ppm [CD2Cl2, 298 K]; 7.23 (HR,γ), 5.97 (Hâ) ppm [CD2Cl2, 193
K]; 7.06 (HR,γ), 5.74 (Hâ) ppm [acetone-d6, 298 K]; 7.00 (HR,γ), 5.64
b
(Hâ) ppm [acetone-d6, 193 K]. Free pyrazole: (0.024 M) CD2Cl2,
193 K 12.53 (1), 7.74 (2), 6.39 (1) ppm; 298 K 9.63 (1), 7.60 (2),
6.40 (1) ppm; acetone-d6 (0.28 M) 193 K 12.94 (1), 7.84 (1, br),
7.69 (1, br), 6.37 (1) ppm; 298 K 12.13 (1), 7.61 (2), 6.28 (1) ppm.
In the latter solvent, the resonances of the pyrazole-acetone
adduct are also observed, as previously reported (Nesmeyanov, A.
N.; Zavelovich, E. B.; Babin, V. N.; Kochetkova, N. S.; Fedin, E. I.
Tetrahedron 1975, 31, 1463).
usual low-field shift caused by N-donor ligands, with
respect to the value of ca. -17 ppm typical of bridging
hydrides in saturated clusters of rhenium containing
carbonyl ligands only.8,9
The reaction occurs with H2 evolution, as indicated
by gas chromatographic analysis, according to eq 1.
[Re3(µ-H)4(CO)10]- + Hpz f
[Re3(µ-H)3(µ-η2-pz)(CO)10]- + H2 (1)
Differently from what was observed in the case of
pyridine, the reaction is not reversible, even under high
H2 pressure (see Experimental Section).
F igu r e 1. ORTEP drawing of the [Re3(µ-H)3(µ-η2-pz)(CO)9-
(Hpz)]- anion (3a ) with partial labeling scheme. Thermal
ellipsoids drawn at the 30% probability level. Hydrogen
atoms were given arbitrary radii. (1a) Perspective view
with the N-H‚‚‚π hydrogen bond and (1b) lateral view to
emphasize the coordination octahedra tilting.
The evolved H2 could arise by interaction of a hydride
with the N-H proton or by reductive elimination of two
hydrides, as shown in Scheme 1 for the reaction with
pyridine. In this case, the mechanism reported in
Scheme 1 was proved by showing that deuterated
pyridine gives an orthometalated derivative containing
two H and one D atoms in the hydridic sites. We,
therefore, tried to study reaction 1 with pyrazole
selectively deuterated in the nitrogen site to verify the
fate of the D atom (evolved as HD or bonded to the
and -11.29 ppm, in a 2:1 ratio, in CD2Cl2 at 298 K) show
that 3 maintains the Cs symmetry of 2. This implies
either free rotation of the Hpz ligand or freezing of a
rotamer with the ligand plane lying on the idealized
mirror. This point will be discussed later. The chemical
shift of the hydride bridging the Re(µ-pz)Re interaction
is substantially unchanged with respect to 2, while the
hydrides bridging the lateral edges show the low-field
shift expected for the replacement of a carbonyl by the
N-donor ligand. A single-crystal X-ray analysis was
performed to establish the syn or anti coordination of
the two aromatic rings with respect to the cluster plane.
The structure of the syn configuration is depicted in
Figure 1 and will be described in detail in the following
discussion.
1
cluster in a “Re3H2D” isotopomer). However, H and
2H NMR analyses of samples taken at different reaction
times showed extensive deuteration of the hydridic sites
of the parent anion 1 itself, thus preventing any
mechanistic inference.
When the reaction of 1 with molten pyrazole was
carried out for longer reaction times, the concentration
of 2 slowly decreased (with a t1/2 of ca. 70 min, see
Experimental Section) and two novel hydridic species
3 and 4 were formed.
Compound 3 has been formulated, on the basis of its
NMR data (see Experimental Section and Table 1), as
the anion [Re3(µ-H)3(µ-η2-pz)(CO)9(Hpz)]- containing
one pyrazolato ligand bridging a cluster edge and a
second pyrazole molecule terminally bonded on the third
vertex of the triangle, replacing an axial carbonyl of 2
(see Scheme 2). The two hydridic resonances (δ -9.90
The formation of 3 clearly arises from the thermal
labilization of an axial carbonyl of the Re(CO)4 vertex
of 2 (eq 2). Also, the reaction of 1 with pyridine gives
[Re3(µ-H)3(µ-η2-pz)(CO)10]- + Hpz a [Re3(µ-H)3
(µ-η2-pz)(CO)9(Hpz)]- + CO (2)
(8) Beringhelli, T.; D’Alfonso, G.; Freni, M.; Ciani, G.; Sironi, A.;
Molinari, H. J . Chem. Soc., Dalton Trans. 1986, 2691.
(9) Beringhelli, T.; D’Alfonso, G.; Panigati, M. J . Organomet. Chem.
1997, 527, 215.
an analogous [Re3(µ-H)3(µ-η2-NC5H4)(CO)9(NC5H5)]- de-
rivative when performed at high temperature (80 °C).

Reactions of Pyrazole with Clusters of Re
Organometallics, Vol. 17, No. 15, 1998 3285
solution, it goes to completion in ca. 3 h, at room
temperature, in the presence of only 10 equiv of pyra-
zole.
NMR monitoring of reaction 4 showed, at the begin-
ning of the experiment, the formation of a novel species
besides anion 3, responsible for two hydridic resonances
at δ -10.80 and -11.82 ppm, in 2:1 ratio. This species
can be reasonably formulated as an isomer of anion 3,
containing the two pyrazole rings in an anti configura-
tion (3b). At the initial stages of the reaction, 3b is
formed in a comparable amount with the syn isomer
(hereafter referred to as 3a ). Its concentration attains
the maximum value (10-15%) at about one-half of the
reaction time, then slowly decreases, leaving 3a as the
unique final product (see Figure S1 in the Supporting
Information). Mixtures of syn and anti isomers have
also been obtained in the reactions of the µ3-imidoyl
cluster [Os3(µ-H)(µ3-η2-CdNC3H6)(CO)9] with amines.12
In this case, it was shown that the syn isomer was the
kinetic product while the thermodynamically favored
species depended on the cone angle of the amine, the
bulkier ligands (such as pyrrolidine) slightly favoring
the isomer containing the µ-imidoyl and the amine on
the same face of the cluster.
F igu r e 2. ORTEP drawing of the [Re2(µ-H)(µ-η2-pz)2-
(CO)6]- anion (4) with partial labeling scheme. Thermal
ellipsoids drawn at the 30% probability level. Hydrogen
atoms were given arbitrary radii.
Reaction 2 is reversible: treatment with CO (1 atm)
results in the transformation of 3 into 2, but the reaction
is very slow (complete after 1 week, at room tempera-
ture).
The third hydridic species observed in the reaction
mixtures in molten pyrazole (4) shows one hydridic and
two pyrazolic resonances in the 1H NMR spectrum (ratio
1:2:4). A single-crystal X-ray analysis formulated this
novel product as the dinuclear anion [Re2(µ-H)(µ-η2-pz)2-
(CO)6]- containing two pyrazolato and one hydrido
ligand bridging the Re-Re interaction (see Figure 2 and
the description of the structure below).
A fourth product has been recognized in the hexane
solution used to separate the NEt4+ salts of the anionic
products from the excess of pyrazole (see Experimental
Section). On the basis of its IR and NMR data, it has
been formulated as the neutral mononuclear complex
[Re(pz)(CO)3(Hpz)2] (5), recently synthesized and char-
acterized.10 Equation 3 formally accounts for the frag-
mentation of the trinuclear cluster 3 to 4 and 5.
In the case of reaction 4, it was possible to perform
experiments with selectively deuterated pyrazole, be-
cause the milder reaction conditions, with respect to
reaction 1, prevented deuteration of the hydridic sites
of the starting complex. NMR monitoring of the reac-
tion of 6 with D-pz clearly showed the appearance of
the pseudotriplet (δ 4.4 ppm, J HD ) 43 Hz) due to free
HD, thus demonstrating that H2 evolved in reaction 4
arises from one hydride and the N-H proton and not
from two H ligands. It was not possible to establish if
the D atom came from free or coordinated pyrazole,
because the easy intermolecular exchange of the protons
on nitrogen did not allow the selective isotopic labeling
of the NH sites.
Other experiments were then designed to differentiate
the two “roles” of Hpz in reaction 4 (coordinated or
entering ligands). With this aim, we pursued the
synthesis of [Re3(µ-H)3(µ-η2-pz)(CO)9L]- derivatives,
containing L ligands different from CO or Hpz, through
two different ways: (i) reaction of [Re3(µ-H)4(CO)9-
(Hpz)]- with L; (ii) reaction of [Re3(µ-H)4(CO)9L]-
complexes with Hpz.
[Re3(µ-H)3(µ-η2-pz)(CO)9(Hpz)]- + 3Hpz f
[Re2(µ-H)(µ-η2-pz)2(CO)6]- + [Re(pz)(CO)3(Hpz)2] +
2H2 (3)
Rea ctivity of oth er [Re3(µ-H)4(CO)9L]- Un sa tu r -
a ted An ion s. A more selective route to anion 3 is
provided by reaction 4, whose starting material is the
unsaturated anion [Re3(µ-H)4(CO)9(Hpz)]- (6, 46 valence
electrons) containing a pyrazole ligand terminally bonded
on a vertex of the triangle (see Scheme 2).11 Reactions
The first route proved to be impracticable with good
L ligands such as CO or PMe2Ph, because they rapidly
substituted the pyrazole ligand. With the poor ligand
MeCN (used in a 10-fold excess) the slow formation of
the expected [Re3(µ-H)3(µ-η2-pz)(CO)9(NCMe)]- anion (7)
was observed (see Experimental Section), but also
accompanied, in this case, by a significant amount of
the substitution derivative [Re3(µ-H)4(CO)9(NCMe)]- (8).
The presence of 8, able to give 7 by reaction with Hpz
(eq 5), does not permit us to say with certainty that 7
derives from reaction 6. However, the concentration of
[Re3(µ-H)4(CO)9(Hpz)]- + Hpz f
[Re3(µ-H)3(µ-η2-pz)(CO)9(Hpz)]- + H2 (4)
1 and 4 are fully analogous, because both involve the
formation of a bridging pyrazolato ligand, with H2
evolution, from the reaction of Hpz with a 46-valence
electron complex. However reaction 4 occurs in much
milder conditions than reaction 1. Indeed, in CH2Cl2
[Re3(µ-H)4(CO)9(NCMe)]- + Hpz f
[Re3(µ-H)3(µ-η2-pz)(CO)9(NCMe)]- + H2 (5)
(10) Ardizzoia, A. G.; La Monica, G.; Maspero, A.; Moret, M.;
Masciocchi, N. Eur. J . Inorg. Chem., in press.
(11) Beringhelli, T.; D’Alfonso, G.; Panigati, M.; Porta, F.; Mercan-
delli, P.; Moret, M.; Sironi, A. Manuscript in preparation.
(12) Kabir, S. E.; Rosenberg, E.; Day, M.; Hardcastle, K.; Wolf, E.;
McPhillips, T. Organometallics 1995, 14, 721.

3286 Organometallics, Vol. 17, No. 15, 1998
Beringhelli et al.
[Re3(µ-H)4(CO)9(Hpz)]- + MeCN f
Ta ble 2. Su m m a r y of Cr ysta l Da ta a n d Str u ctu r e
Refin em en t P a r a m eter s for 3a a n d 4
[Re3(µ-H)3(µ-η2-pz)(CO)9(NCMe)]- + H2 (6)
3a
4
formula
fw
cryst syst
space group
a, Å
C40H32Cl2N4O9PRe3 C36.5H28ClN4O6PRe2
free pyrazole present in the reaction mixture must be
considered very low, the upper limit being the amount
of 8. This is confirmed by the very small amount of 3
simultaneously formed by reaction of 6 with Hpz (see
Experimental Section). Moreover, the time course of the
formation of 7 shows the absence of the “induction
period”, which is typical of consecutive reactions. We
can, therefore, be confident that reaction 6 does occur.
1373.17
triclinic
P1h (No. 2)
9.239(1)
14.050(2)
17.688(2)
91.49(6)
91.56(2)
103.85(5)
2227.2(5)
2
1057.45
monoclinic
C2/c (No. 15)
37.584(1)
10.102(1)
21.536(2)
b, Å
c, Å
R, deg
â, deg
114.03(1)
γ, deg
V, Å3
7468(1)
8
The nature of anion 7 has been established from its
1H NMR data: the two hydridic resonances, in a 1:2
ratio, are at δ values in line with the presence of three
coordinated N atoms (-10.79 and -11.66 ppm, 2:1, CD2-
Cl2). The two signals of the pyrazolate (2:1 ratio)
confirm the Cs symmetry of the anion, while the
resonance of MeCN (δ 1.78 ppm) appears upfield with
respect to the free ligand. This could be attributable to
the shielding effect of the π-electron cloud of the
bridging pyrazolate, as previously observed for the
dominant isomer of the related [Re3(µ-H)3(µ-η2-NC5H4)-
(CO)9(NCMe)]- anion.9 On these grounds, 7 should be
identified as the syn isomer. There is no evidence for
the presence of an anti isomer, though it could be
responsible of some of the very minor signals observed
in the reaction mixture.
As to the second route to [Re3(µ-H)3(µ-η2-pz)(CO)9L]-
anions, the fast substitution of pyridine with pyrazole
made impossible the use of [Re3(µ-H)4(CO)9(Py)]-, which
could be a very good model of the anion 6. Thus, we
moved to the unsaturated complex [Re3(µ-H)4(CO)9(PMe2-
Ph)]- that contains a less labile, but still good σ-donor
ligand. In this case, the clean formation of two isomers
of the mixed pyrazolato-phosphine derivative [Re3(µ-
H)3(µ-η2-pz)(CO)9(PMe2Ph)]- (9) was observed. The two
species, assigned on the basis of the pattern of the
hydridic resonances to an axial (9a ) and equatorial (9b)
isomer, respectively, are always in ca. 4:1 ratio, which
likely represents their thermodynamic ratio. In fact,
the rate of reaction 7 was low, much lower than that of
reaction 4 (ca. 40% of conversion after 24 h, in the same
conditions of reaction 4, i.e., in CD2Cl2, at 298 K, in the
presence of 10 equiv of pyrazole), but faster than
reaction 1 in the same conditions (no reaction after 24
h).
Z
temp, K
abs coeff, mm-1
no. indep reflns,
R(int)
223(2)
8.344
223(2)
6.642
9474, 0.0241
8308, 0.0279
R1, wR2 (Fo
>
0.0197, 0.0382
0.0210, 0.0391
0.0331, 0.0415
4σ(Fo))a
R1, wR2 (all data) 0.0280, 0.0393
Ta ble 3. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for 3a a n d 4
3a
4
Re(1)-Re(2)
Re(1)-Re(3)
Re(2)-Re(3)
Re(1)-C(11)
Re(1)-C(12)
Re(1)-C(13)
Re(2)-C(21)
Re(2)-C(22)
Re(2)-C(23)
Re(3)-C(31)
Re(3)-C(32)
Re(3)-C(33)
Re(1)-N(1)
Re(2)-N(3)
Re(3)-N(4)
3.259(1) Re(1)-Re(2)
3.277(1) Re(1)-C(11)
3.217(1) Re(1)-C(12)
1.905(4) Re(1)-C(13)
1.913(4) Re(2)-C(21)
1.924(4) Re(2)-C(22)
1.919(4) Re(2)-C(23)
1.922(4) Re(1)-N(1)
1.908(4) Re(2)-N(2)
1.908(4) Re(1)-N(3)
1.914(4) Re(2)-N(4)
1.913(4)
3.2380(2)
1.921(3)
1.926(3)
1.923(3)
1.925(3)
1.923(3)
1.929(4)
2.166(2)
2.163(2)
2.156(2)
2.170(2)
2.209(3)
2.161(3)
2.175(3)
Re(1)-Re(2)-Re(3) 60.79(2) C(11)-Re(1)-C(12) 89.8(1)
Re(1)-Re(3)-Re(2) 60.24(4) C(11)-Re(1)-C(13) 89.6(1)
Re(2)-Re(1)-Re(3) 58.97(3) C(12)-Re(1)-C(13) 89.4(1)
C(11)-Re(1)-C(12) 90.8(2)
C(11)-Re(1)-C(13) 87.6(2)
C(12)-Re(1)-C(13) 89.8(2)
C(11)-Re(1)-N(1)
C(12)-Re(1)-N(1)
C(13)-Re(1)-N(1)
C(21)-Re(2)-C(22) 89.4(2)
C(21)-Re(2)-C(23) 87.4(2)
C(22)-Re(2)-C(23) 91.6(2)
C(21)-Re(2)-N(3)
C(22)-Re(2)-N(3)
C(23)-Re(2)-N(3)
C(31)-Re(3)-C(32) 89.8(2)
C(31)-Re(3)-C(33) 88.9(2)
C(32)-Re(3)-C(33) 87.5(2)
C(31)-Re(3)-N(4)
C(32)-Re(3)-N(4)
C(33)-Re(3)-N(4)
C(11)-Re(1)-N(3)
C(12)-Re(1)-N(3)
C(13)-Re(1)-N(3)
C(11)-Re(1)-N(1)
C(12)-Re(1)-N(1)
95.2(1)
174.0(1)
94.0(1)
96.0(1)
94.41)
93.3(2)
88.6(1)
178.2(1) C(13)-Re(1)-N(1)
173.3(1)
81.77(8)
N(1)-Re(1)-N(3)
C(21)-Re(2)-C(22) 90.7(1)
C(21)-Re(2)-C(23) 89.7(1)
C(22)-Re(2)-C(23) 87.1(1)
95.3(2)
91.8(1)
C(21)-Re(2)-N(2)
94.6(1)
94.9(1)
175.2(1)
93.7(1)
175.1(1)
94.9(1)
82.69(8)
175.7(1) C(22)-Re(2)-N(2)
C(23)-Re(2)-N(2)
C(21)-Re(2)-N(4)
C(22)-Re(2)-N(4)
C(23)-Re(2)-N(4)
N(2)-Re(2)-N(4)
[Re3(µ-H)4(CO)9(PMe2Ph)]- + Hpz f
[Re3(µ-H)3(µ-η2-pz)(CO)9(PMe2Ph)]- + H2 (7)
94.7(2)
94.2(1)
176.0(1)
Descr ip tion of th e Str u ctu r es of An ion s 3a a n d
4. Crystals of the PPh4+ salt of anion 3a consist of the
packing of discrete anions and cations plus clathrated
CH2Cl2 molecules in a 1:1:1 ratio, respectively, held
together by Coulombic and van der Waals interactions.
An ORTEP drawing of the molecular structure of 3a is
reported in Figure 1. Relevant bond parameters are
listed in Table 3.
cluster [Re3(µ-H)4(CO)10]-,13 one axial CO of the Re(CO)4
unit is actually replaced by a monodentate N-coordi-
nated pyrazole while the Re-Re edge originally bridged
by two hydrides is now spanned by a hydride and a µ-η2-
pyrazolato group, acting as an exo-bidentate ligand.
When considering the bonding interactions around the
Re atoms, each metal attains a distorted octahedral
coordination given by three CO’s, two hydrides, and one
Anion 3a possesses a triangular metal core with all
edges bridged by one hydrido ligand. Each of the
rhenium atoms also bears three terminal CO ligands
with facial coordination. With reference to the parent
(13) Beringhelli, T.; Ciani, G.; D’Alfonso, G.; Molinari, H.; Sironi,
A. Inorg. Chem. 1985, 24, 2666.

Reactions of Pyrazole with Clusters of Re
Organometallics, Vol. 17, No. 15, 1998 3287
nitrogen atom, if direct metal-metal interactions are
neglected. Cluster 3a shows two long Re-Re distances
(Re(1)-Re(2) 3.259(1) Å and Re(1)-Re(3) 3.277(1) Å)
and one shorter interaction (Re(2)-Re(3) 3.217(1) Å)
bridged by the pyrazolato ligand; a similar pattern has
been observed in the related [Re3(µ-H)3(µ-η2-NC5H4)-
(CO)9(NC5H5)]- (NC5H5 ) pyridine) cluster1 where
pyridine substitutes for pyrazole. Compared to [Re3(µ-
H)4(CO)10]-, the metal framework in 3a is enlarged
significantly; the most evident distance expansion,
involving Re(2)-Re(3) increasing from 2.789(1) to 3.217-
(1) Å, is due to the removal of the formal double Re-Re
bond, while the remaining two edges, which are en-
larged by about 0.08 Å, suffer from the steric presence
of the pyrazole pointing toward the µ-η2-pyrazolate. As
anticipated in the previous sections, the two pyrazole
moieties lie on the same side of the Re3 triangle giving
rise to a syn conformation and are involved in a N-H‚
‚‚π-electrons hydrogen bond. The two pyrazolic rings
are almost orthogonal to each other (dihedral angle 83.4-
(2)°), in accordance with a significant interaction be-
tween the N(2)-H(2N) group, the π-electron cloud of the
pyrazolato ligand, and an overall Cs idealized symmetry
(in the related [Re3(µ-H)3(µ-η2-NC5H4)(CO)9(NC5H5)]-
anion the two heterocyclic ligands are syn but face each
other in an almost parallel fashion).1 H(2N) to µ-pz
distances range from about 2.22 and 2.36 Å for N(3) and
N(4), respectively, to 2.40, 2.61, and 2.64 Å for C(4), C(6),
and C(5), respectively, with a H(2N)-pz(centroid) dis-
tance of 2.15 Å and a N(2)-H(2N)-pz(centroid) angle
of 165°. Recently,14 intermolecular N-H‚‚‚π hydrogen-
bonded systems have received attention owing to their
relevant contribution to crystal packing motives. In
such systems, the average distance between H and the
π-cloud centroid is about 2.45 Å, suggesting that the
N-H‚‚‚π intramolecular interaction in 3a is strong
enough to stabilize the orthogonal versus the parallel1
conformation of the aromatic ligands and to offer further
reasons for the preference for the syn versus the anti
isomer.
Crystals of 4 consist of the packing of [Re2(µ-H)(µ-η2-
+
pz)2(CO)6]- anions and PPh4 cations together with
clathrated CH2Cl2 molecules in a 1:1:0.5 ratio, respec-
tively. The molecular structure of 4 is shown in Figure
2, together with a partial labeling scheme. The di-
nuclear anion contains two fac-Re(CO)3 units linked by
one hydrido and two pyrazolato bridging ligands (34
valence electrons), giving rise to a distorted octahedral
coordination around the Re centers. The two µ-η2-pz
units are almost orthogonal to each other (dihedral
angle between least-squares planes 89.3(1)°) with the
Re-[N-N]2-Re metallacycle adopting a boat conforma-
tion, giving a C2v idealized symmetry for this dimeric
species.
Dimeric rhenium(I) compounds bridged by pyrazolate
are a novel class of compounds, the only other examples
known being the 36-valence electron compounds [Re2-
(µ-X)(µ-η2-pz)2(CO)6]- (X ) OH, Br).10 In the past,
pyrazolato ligands coordinating in an exo-bidentate
mode15 have shown a great coordination versatility
regarding the spanned metal-metal distance, the N-N-
metal bond angles, and the off-plane displacement of
the coordinated metals. In the present case, the Re-
Re distance of 3.2380(2) Å is comparable to that
observed in the dimeric 34-valence electron compounds
with a bridging orthometalated pyridine [Re2(µ-H)(µ-
η2-C5NH4)(CO)7L] (L ) CO, (CH3)3NO),16 which exhibit
values ranging from 3.195 to 3.232 Å. On the contrary,
the 36-valence electron compounds [Re2(µ-OH)(µ-η2-pz)2-
(CO)6]- and [Re2(µ-Br)(µ-η2-pz)2(CO)6]- have Re‚‚‚Re
distances of 3.660(1) and 3.825(1) Å,10 respectively, as
expected well beyond the accepted range of bond inter-
actions. To alleviate these relevant variations in the
Re-Re distance, pyrazolato ligands are able to change
their bonding geometry, namely the Re-N distances
and N-N-Re angles. For the former parameter there
is no significant difference between the µ-H and µ-OH
species (average 2.164 and 2.163 Å, respectively), while
the bromide derivative shows an average value of 2.174
Å. More sensitive to the nature of the bridging group
are the N-N-Re angles, which display a regular trend
on moving from µ-H (115.5°) to µ-OH (120.0°) and µ-Br
(124.6°). Another feature of the µ-pyrazolato coordina-
tion capabilities is the twist of the pz ring with respect
to the Re-Re dumbbell. In 4, the off-plane displace-
ment of the rhenium atoms ranges from +0.050(5) (Re-
(1)) and -0.226(5) Å (Re(2)) with respect to pz(1-2) to
+0.087(5) (Re(1)) and -0.076(5) Å (Re(2)) with respect
to pz(3-4). Similar values have been observed in group
11 metal pyrazolates.15c
Hydrides H(1) and H(3) are displaced about 0.5 Å off
the metal triangle on the same side of the pyrazole
moieties, while the hydride H(2) lies opposite µ-pz, about
0.9 Å off the metal plane. Owing to the presence of the
intramolecular hydrogen bond involving the two het-
erocycles, which imposes their orthogonal conformation,
the coordination octahedra at the Re atoms are signifi-
cantly tilted with respect to the Re3 plane, as outlined
in Figure 1b and by the values of the angle between
the Re3 plane, the Re(1)-N(1) vector (100.74(8)°), and
the bridging pyrazolate (105.0(1)°). On the contrary, in
[Re3(µ-H)3(µ-η2-NC5H4)(CO)9(NC5H5)]-, owing to the
parallel conformation of the heterocyclic ligands and the
consequent decrease of the steric crowding, the average
Re-Re-N(terminal) angles amount to 94.0° (99.3° in
3a ) and the Re3/µ-NC5H4 dihedral angle is 93.5°. Re-N
bond lengths in 3a are normal and show increased
values for the axial pyrazole (Re(1)-N(1) ) 2.209(3) Å)
with respect to the bridging pyrazolato unit (Re(2)-N(3)
) 2.161(3) Å, Re(3)-N(4) ) 2.175(3) Å).
In both 3a and 4 the carbonyl ligands are linear with
Re-C-O angles between 175.4(3)° and 179.7(3)°. All
pyrazolato moieties in anions 3a and 4 are planar within
experimental errors, the maximum RMS out of plane
deviation being 0.003 Å.
Th e N-H‚‚‚π-Electr on s Hyd r ogen Bon d . “Non-
classic” hydrogen bond interactions involving π-electrons
as an acceptor have been previously recognized and
(15) (a) Ardizzoia, A. G.; Angaroni, M. A.; La Monica, G.; Cariati,
F.; Cenini, S.; Moret, M.; Masciocchi, N. Inorg. Chem. 1991, 30, 4347.
(b) Ardizzoia, A. G.; Beccalli, E. M.; La Monica, G.; Masciocchi, N.;
Moret, M. Inorg. Chem. 1992, 31, 2706. (c) Ardizzoia, A. G.; Cenini,
S.; La Monica, G.; Masciocchi, N.; Moret, M. Inorg. Chem. 1994, 33,
1458.
(14) (a) Hanton, L. R.; Hunter, C. A.; Purvis, D. H. J . Chem. Soc.,
Chem. Commun. 1992, 1134. (b) Allen, F. H.; Hoy, V. J .; Howard, J .
A. K.; Thalladi, V. R.; Desiraju, G. R.; Wilson, C. C.; McIntyre, G. J . J .
Am. Chem. Soc. 1997, 119, 3477.
(16) Nubel, P. O.; Wilson, S. R.; Brown, T. L. Organometallics 1983,
2, 515.

3288 Organometallics, Vol. 17, No. 15, 1998
Beringhelli et al.
described (see for instance refs 14 and 17). To the best
of our knowledge (substantiated by a thorough search
through the October 1997 release of the Cambridge
Crystallographic Database for similar intra and inter-
molecular interactions), there are no previous examples
of structurally characterized hydrogen bonds involving
compounds with the π-electrons of nitrogen-heterocycles
as acceptors (while phenyl π-electrons are already
known14) and N-H groups as donors.
The NMR data provide evidence that this intramo-
lecular hydrogen bond is maintained in solution and
features the dominant conformation in a noncoordinat-
ing solvent such as CH2Cl2.
The N-H protonic resonance of 3a is observed in CD2-
Cl2 at 193 K at δ 7.12 (δ 7.78 at 298 K), which is
significantly upfield with respect to the range of values
usually found in complexes containing terminally bonded
Hpz (δ values between 12 and 9 ppm, at room temper-
ature, in CDCl3 or CD2Cl2).3c,17,18 In anion 6, for
instance, the N-H resonance is at δ 10.1 (CD2Cl2,
independent of the temperature). Inter- or intramo-
lecular classical hydrogen bonds can move this signal
at still higher frequencies, as observed for free pyrazole,
involved in extensive intermolecular N-H‚‚‚N interac-
tions. The high-field shift observed for 3a is attributable
to the shielding effect of the electronic π cloud of
coordinated pyrazolate and, therefore, indicates that
also in solution the N-H proton is pointing toward the
center of the aromatic ring. In agreement with this, the
species 3b, which has been formulated as the anti
isomer of 3a , shows its NH resonance at a normal δ
value (10.60 ppm) under the same conditions.
F igu r e 3. Contour plot of the negative levels of a 1H 2D
NOESY phase-sensitive experiment performed on a solu-
tion of [NEt4]3a (298 K, CD2Cl2, 7.05 T).
the R hydrogens of the pyrazolate moiety (at δ 7.33 ppm)
and the other, stronger, with the signal at δ 6.91 ppm
of the pyrazole. This last resonance displays strong
through-space connectivities, also with the signals at
6.11 (Hâ of Hpz) and 6.05 ppm (Hâ of pz). Less intense
correlations are observed with HR of the pyrazole and
with the signal at 7.68 ppm (Hγ of Hpz). The conforma-
tion observed in the solid state is, therefore, maintained
in solution, even at room temperature.
The exoergonic nature of this H-bond interaction
could be assumed as responsible for the dominance of
the syn isomer 3a over 3b. An unconventional Os-H‚
‚‚H-N interaction has been shown to stabilize the syn
isomers of the [Os3H(µ-H)(CO)10L] complexes (L ) NH2-
Et, NHEt2).21 However, in the related [Re3(µ-H)3(µ-η2-
NC5H4)(CO)9(NC5H5)]- anion, where H-bonding cannot
occur, only the syn isomer was found.1 Lower steric
interligand repulsions could, therefore, be responsible
in both cases for the syn configuration.
In the presence of a solvent able to act as an
hydrogen-bond acceptor, such as acetone, intermolecular
N-H‚‚‚solvent interactions replace the intramolecular
H bond described above. The NMR data (see Table 1)
indeed suggest that at low temperature in acetone anion
3a adopts conformations in which the pyrazole ligand
is hydrogen bound to the solvent. At 193 K, the NH
resonance is now observed at δ 12.2, a value quite
similar to that of free pyrazole in the same conditions
(δ 12.94), thus showing the disappearance of the π-elec-
tron shielding effect and the involvement of NH in
normal hydrogen bonds with acetone. The three reso-
nances of the C-H’s of the pyrazole occur at δ values
close to those found in CD2Cl2 (see Table 1). However,
in the present case, 1D NOE experiments (see Experi-
mental Section) have shown that the positions of the
Indeed, the occurrence of an upfield shift has been
previously used to reveal intermolecular π-hydrogen
bonds, such as those between the C-H group of chlo-
roform and the cyclopentadienyl rings of ferrocenes.19
Moreover, the shift at lower field undergone by the N-H
resonance of pyrrole on dilution with an “innocent”
solvent such as cyclohexane has also been attributed to
a decrease of the intermolecular π-interactions between
the N-H proton and the π-electron ring of a second
molecule.20 A similar shift, though considerably reduced
in magnitude, was also observed for the R-CH protons
1
of pyrrole. In anion 3a , one of the H NMR resonances
of the C-H groups of pyrazole appears shifted upfield
(δ 6.89 ppm, see Table 1) with respect to the typical
values of pyrazole. On this basis, this signal could be
attributed to the proton on the C atom R to NH (C3 in
Figure 1), which is located at the periphery of the
screening region of the π-electron density.
A 2D-NOESY experiment (Figure 3) at 298 K has
confirmed this assignment. Indeed, the broad signal of
the NH proton (δ 7.78) has two cross-peaks: one with
(17) (a) Atwood, J . L.; Hamada, F.; Robinson, K. D.; Orr, G. W.;
Vincent, R. L. Nature 1991, 349, 683. (b) Cavaglioni, A.; Cini, R. J .
Chem. Soc., Dalton Trans. 1997, 1149. (c) Bakke, J . M.; Chadwick, D.
J . Acta Chem. Scand. 1988, B42, 223. (d) Kazaryan, S. G.; Lokshin, B.
V.; Kimel’fel’d, Y. M.; Materikova, R. B. Izv. Akad. Nauk SSSR, Ser.
Khim. 1986, 11, 2603.
(18) (a) Alberte, B.; Sanchez Gonzalez, A.; Garcia, E.; Casas, J . S.;
Sordo, J .; Castellano, E. E. J . Organomet. Chem. 1988, 338, 187. (b)
Esteruelas, M. A.; Lahoz, F. J .; Lopez, A. M.; Onate, E.; Oro, L. A.;
Ruiz, N.; Sola, E.; Tolosa, J . I. Inorg. Chem. 1996, 35, 7811.
(19) Cerichelli, G.; Illuminati, G.; Ortaggi, G.; Giuliani, A. M. J .
Organomet. Chem. 1977, 127, 357.
(20) See Happe, J . A. J . Phys. Chem. 1961, 65, 72 and references
therein.
(21) Aime, S.; Gobetto, R.; Valls, E. Organometallics 1997, 16, 5140.

Reactions of Pyrazole with Clusters of Re
Organometallics, Vol. 17, No. 15, 1998 3289
process is reminiscent of the reaction of 1 with I2 or HI
in nondonor solvents,22 which resulted in the stepwise
fragmentation of the cluster skeleton by progressive
substitution of the Re(µ-H)Re interactions by Re(µ-I)-
Re ones. In particular, a dinuclear [Re2(µ-H)(µ-I)2-
(CO)6]- anion, similar to 4, was isolated and structurally
characterized.
The reagents used in that case were strong electro-
philes, able to attack the hydridic sites of the cluster
anions, and the overall fragmentation occurred instan-
taneously, at room temperature, with stoichiometric
amounts of reagents. In the present case, the ampho-
teric nature of pyrazole could suggest mechanisms based
either on electrophilic or nucleophilic attack on 1. The
drastic conditions (molten pyrazole as solvent) required
even for the first reaction step (eq 1) could be in line
either with the very low electrophilicity of pyrazole (pKa
of the “pyrrolic” N-H bond 14.2)4 or with the poor
propensity of 1 to react with nucleophiles, despite its
electronic unsaturation.23 The reaction with pyridine,
for instance, at room temperature occurs at a reasonable
rate only when the ligand is in large excess (typically
when it is used as solvent).1
Some clue is provided by the comparison of the rate
of the reactions with Hpz of different 46-valence electron
complexes [Re3(µ-H)4(CO)9L]-. The increase of the rate
in the order L ) CO < PMe2Ph < Hpz corresponds to
an increase of electron density on the cluster (and of
“hydridic” character of the H ligands), on the basis of
the known π-acceptor and σ-donor properties of the
involved ligands. This, therefore, rules out that pyra-
zole coordination could be the rate-determining step and
is in agreement with the idea of an electrophilic attack.
F igu r e 4. Contour plots of selected sections of a 1H 2D
NOESY phase-sensitive experiment performed on a solu-
tion of [NEt4]3a (298 K, acetone-d6, 7.05 T). Only the
negative levels have been plotted. The region of the CH
pyrazolic resonances is shown in F2, while in F1 the
sections of NH (bottom) and the CH pyrazolic resonances
(top) are reported.
signals of HR and Hγ are exchanged and the screening
effect of the bridging pyrazolate ligand is now acting
on Hγ while the resonance of HR is at normal δ values.
The pyrazole molecule is, therefore, rotated with respect
to the conformation observed in CD2Cl2.
However, H2 evolution from intermolecular pz-Hδ+
-
Hδ--Re interactions seems unlikely, due to the poor
acidity of free Hpz. Reaction 9, in fact, has never been
observed with acids weaker than phenol.24 Moreover,
On raising the temperature, the N-H signal of 3a
exhibits an upfield shift much higher than that observed
for free pyrazole in the same conditions (see Table 1).
The same behavior is observed for the signal of HR, while
that of Hγ undergoes a shift in the opposite direction
(so that at room-temperature HR and Hγ become almost
isochronous, ∆δ 0.05 ppm). These assignments have
been confirmed by a 2D NOESY experiment at 298 K
(Figure 4) that showed the correlation of the NH proton
with the resonance at lower field (δ 7.30), thus assigned
to HR. This experiment has also shown that the HR,γ
protons of the bridging pyrazolate interact with three
different protons of the terminal Hpz, namely HR, Hγ,
and NH. This indicates that different conformations are
populated at room temperature and that they are in fast
exchange. The δ values of the protonic resonances are,
therefore, averaged values of the signals of the different
conformers, and the shifts of the signals of HR and Hγ
with the temperature arise from the change of their
relative populations.
[Re3(µ-H)4(CO)10]- + HX f
[Re3(µ-H)3(µ-X)(CO)10]- + H2 (9)
(X ) Cl, Br, I, RCO2, ArO)
anion 6, in the absence of added ligands, is stable in
solution for a few hours, despite possessing H ligands
more hydridic than those of 1 and a pyrazole ligand
more acidic than free Hpz (due to the coordination).
It seems, therefore, more likely that the Hδ+-Hδ-
interaction occurs concertedly with pyrazole coordina-
tion25 or after this coordination, in a saturated (and
therefore more electron rich) intermediate I1 of Scheme
3, containing a terminal hydride (more reactive) and a
coordinated Hpz (more acidic). This intermediate could
be formed (possibly in a fast preequilibrium step) not
(22) Ciani, G.; D’Alfonso, G.; Romiti, P.; Sironi, A.; Freni, M. Inorg.
Chem. 1983, 22, 3115.
(23) See, for instance: Beringhelli, T.; Ciani, G.; D’Alfonso, G.; Freni,
M. J . Organomet. Chem. 1986, 311, C51.
(24) Beringhelli, T.; Ciani, G.; D’Alfonso, G.; Sironi, A.; Freni, M. J .
Chem. Soc., Dalton Trans. 1985, 1507.
F in a l Discu ssion . The overall reaction of 1 with
pyrazole (eq 8) leads to fragmentation of the triangular
cluster, with formal replacement of three hydrides by
three pyrazolate anions. From this point of view, the
(25) The concerted electrophilic/nucleophilic attack of HX acids on
the µ3-imidoyl cluster [Os3(µ-H)(µ3-η2-CdNC3H6)(CO)9] has been previ-
ously discussed, see: Gobetto, R.; Hardcastle, K.; Kabir, S. E.; Milone,
L.; Nishimura, N.; Botta, M.; Rosenberg, E.; Yin, M. Organometallics
1995, 14, 3068.
[Re3(µ-H)4(CO)10]- + 5Hpz f [Re2(µ-H)(µ-pz)2(CO)6]-
[Re(pz)(CO)3(Hpz)2] + 3H2 + CO (8)
+

3290 Organometallics, Vol. 17, No. 15, 1998
Beringhelli et al.
Exp er im en ta l Section
Sch em e 3
The reactions were performed under N2, and solvents were
dried and deoxygenated by standard methods. Pyrazole
(Fluka) was used as received. Published methods were used
for the syntheses of [NEt4][Re3(µ-H)4(CO)10]13 and [NEt4]-
[Re3H4(CO)9].26 The [Re3(µ-H)4(CO)9(Hpz)]- and [Re3(µ-
H)4(CO)9(PMe2Ph)]- anions were obtained quantitatively by
treating [Re3H4(CO)9]- with the stoichiometric amount of the
proper ligand. The NMR spectra were acquired on Bruker
WP80, AC200, or DRX300 and Varian Gemini 200 spectrom-
eters. Infrared spectra were obtained on a Perkin-Elmer 781
grating spectrophotometer or with a Bruker Vector 22 FT
instrument.
Rea ction s of [NEt4][Re3(µ-H)4(CO)10] ([NEt4]1) w ith
P yr a zole a t 80 °C. Different Schlenk tubes containing the
same amount of [NEt4]1 (typically 8 mg, 0.008 mmol) and
pyrazole (400 mg, 5.9 mmol), without any solvent, were
maintained into a thermostat at 80 °C for different times. In
each sample, pyrazole completely melted within about 1 min,
giving yellow solutions. The reactions were quenched by
introducing the samples into an ice bath. The gas-chromato-
graphic analysis of the evolved gas showed the presence of H2
and (at longer reaction times) CO. The addition of water gave
a pale yellow precipitate, which was dried under vacuum. In
the samples corresponding to the longer reaction times,
treatment of the precipitate with n-hexane allowed the extrac-
tion of the neutral complex [Re(pz)(CO)3(Hpz)2] (5) (identified
by its IR and NMR data).10 NMR analysis of the residue
showed the presence of anions 2, 3a , and 4, in different ratios.
After 5 min, 2 was the main product (2 0.94, 3 0.06), then its
molar fraction slowly decreased (10 min, 2 0.87; 3 0.13) and
the hydridic resonance of 4 appeared (30 min 2 0.82, 3 0.15, 4
0.03; 60 min 2 0.55, 3 0.30, 4 0.15; 120 min 2 0.20, 3 0.44, 4
0.36). At very long reaction times, 4 became the main species
(18 h 3 0.58, 4 0.42; 24 h 3 0.35, 4 0.65). Crystals of the [NEt4]+
salt of anion 4 suitable for X-ray analysis were grown by slow
diffusion of n-hexane into a concentrated CH2Cl2 solution. IR
data: [NEt4]2 (CH2Cl2) ν(CO) 2098w, 2025m, 2006vs, 1985sh,
1943mw, 1910s(br) cm-1; [NEt4]3a (CH2Cl2), ν(CO) 2034sh,
2005vs, 1992sh, 1912s cm-1, ν(N-H) 3340 cm-1, ∆ν1/2 53 cm-1
(cfr. 3460 cm-1, ∆ν1/2 27 cm-1 for free pyrazole in the same
conditions, ca. 3 × 10-2 M); [NEt4]4 (acetone) ν(CO) 2019w,
only by addition of pyrazole to a [Re3(µ-H)4(CO)9L]-
complex, but also by addition of one L ligand to [Re3(µ-
H)4(CO)9(Hpz)]-. The reaction of 6 with MeCN (eq 6)
demonstrates the feasibility of this second route. The
evolution of HD in the reaction of 6 with D-pz agrees
with path b of Scheme 3, and it is reasonable to assume
that the same mechanism is also operative for the other
[Re3(µ-H)4(CO)9L]- complexes.
2000s, 1992sh, 1898vs cm-1
.
1H NMR data (acetone-d6, 295
The formation of the pyrazolate anion in reactions 1,
4, 6, and 7 indicates that only the N-H bond is
activated. Differently from what was observed in the
addition of pyrazole on the Os3(CO)10 fragment3c,7 or in
the reaction of 1 with pyridine,1 no species arising from
the oxidative addition of the ortho C-H bond has been
observed. This different behavior is easily rationaliz-
able on the basis of Scheme 3. H2 elimination from two
hydrides of the intermediate I1 (path a) would give the
H-unbridged Re-Re interaction, necessary in order to
allow the competitive oxidative addition of N-H or C-H
bonds. However, this does not occur because of the
faster reaction between one hydride and the acidic H-pz
proton.
K): [NEt4]2, δ 7.20 (d, 2, HR,γ, J HH 2.0 Hz), 5.85 (t, 1, Hâ, J HH
2.0 Hz), -11.56 (s, 1), -13.59 (s, 2); [NEt4]3a , δ 10.25 (s, 1,
N-H), 7.30 (m, 1, HR of Hpz), 7.25 (m, 1, Hγ of Hpz), 7.06 (m,
2, HR of pz), 6.04 (m, 1, Hâ of Hpz), 5.74 (m, 1, Hâ of pz), -9.69
(s, 2), -11.16 (s, 1); [NEt4]4, δ 7.30 (d, 4, HR,γ, J HH 2.0 Hz),
5.86 (t, 2, Hâ, J HH 2.0 Hz), -8.50 (s, 1). Elemental analysis
has been performed on a sample isolated after a reaction time
of ca. 4 min and, therefore, was constituted mainly of [NEt4]2
(contaminated by a very small amount of 3). Anal. Calcd for
C
21H26N3O10Re3: C, 24.3; H, 2.52; N, 4.04. Found: C, 24.4;
H, 2.7; N, 4.5.
Rea ction of [NEt4]1 w ith D-p z a t 80 °C. The vessels used
for this experiment have been treated with D2O and then
maintained in an oven at 120 °C overnight. A sample of [NEt4]-
1 (10.3 mg, 0.0106 mmol) was mixed without any solvent with
D-pz (351 mg, 5.15 mmol, obtained by maintaining H-pz in 1
mL of D2O for ca. 60 h). After 4 min at 80 °C, the reaction
was quenched and the solid material was dissolved in THF.
Addition of n-hexane caused the precipitation of a pale yellow
solid, shown by NMR analysis to contain a mixture of unre-
acted 1 (about 42%), 2 (about 40%), and 3a (about 17%).
Comparison of the integrated intensities of the resonances in
the hydridic and pyrazole regions indicated the occurrence of
extensive deuteration (more than 50%) in the hydridic sites.
The reaction of 1 with pyrazole shows, therefore,
features intermediate between the reaction with pyri-
dine (coordination of the basic nitrogen) and those with
HX acids (H2 elimination from oppositely polarized H
atoms), in line with the amphiprotic nature of pyrazole.
This also confirms that rhenium hydrido-carbonyl
cluster are less suitable than their isoelectronic osmium
counterpart in activating C-H bonds, due to the pres-
ence of hydrogen bridges on the Re-Re interactions.
(26) Beringhelli, T.; D’Alfonso, G.; Garavaglia, M. G. J . Chem. Soc.,
Dalton Trans. 1996, 1771.

Reactions of Pyrazole with Clusters of Re
Organometallics, Vol. 17, No. 15, 1998 3291
This was confirmed by a 2H NMR spectrum (acetone, 295 K),
which showed substantially homogeneous deuteration in
anions 1, 2, and 3a and in the different hydridic sites.
Attem p ted Rea ction of [NEt4]2 w ith H2. A tube con-
taining a CH2Cl2 solution of [NEt4]2 (0.018 M) was maintained
in an autoclave under 80 atm of H2 at room temperature for 6
days. The solution was evaporated to dryness, and the residue,
analyzed by NMR in deuterioacetone, was still the starting
material.
Rea ction of [NEt4]3a w ith CO. A solution of [NEt4]3a
(12 mg, 0.0111 mmol) in THF (3 mL) was saturated with CO
at 193 K. The sample was then maintained under CO
atmosphere for 6 days. NMR analysis (acetone-d6, 298 K) of
the residue showed the complete transformation of the starting
material into the anion 2.
Hz), -11.66 (d, 1H, J HP 3.0 Hz), -11.98 (d, 2H, J HP 18.6 Hz)
ppm. 1H NMR data of 9b (CD2Cl2, 298 K): δ 7.22 (d, 1H, Hγ/R
,
J HH 1.85 Hz), 7.20 (d, 1H, HR/γ, J HH 1.85 Hz), 5.85 (t, 1H, Hâ,
J HH 1.85 Hz), 2.13 (d, 6H, J HP 9 Hz), -11.78 (s, 1H), -13.07
(d, 1H, J HP 17.1 Hz), -13.30 (s, 1H) ppm.
Va r ia ble-Tem p er a tu r e Sp ectr a a n d n Oe Mea su r em en t
of [NEt4]3a . A sample of [NEt4]3a (19.1 mg, 0.018 mmols)
was dissolved in acetone-d6 (0.5 mL) into a NMR tube, and
1H NMR variable-temperature spectra were acquired on a
Bruker AC 200 spectrometer. The same sample was also used
for nOe difference experiments performed at 193 K. Satura-
tion radio-frequency fields have been applied for 300 ms on
and off resonance with respect to the selected signal. The
acquisitions have been cycled until a good signal-to-noise was
achieved in the spectra obtained through the Fourier trans-
form of the difference of the fids. Irradiation of the NH signal
resulted in an enhancement of the signal at 7.64 ppm. When
this last signal was irradiated, enhancements at 12.19 and 6.07
ppm were observed. Control experiments have been performed
by irradiation of the signals at 6.07 and 6.61 ppm: positive
nOe were observed at 7.64 and 6.61 ppm, and at 6.07 ppm
respectively. A 2D-NOESY phase-sensitive27 experiment was
performed, in the same solvent, on a Bruker DRX 300 at 298
K using a mixing time of 2.0 s. Data points (1K) have been
collected over a spectral width of 1800 Hz for each of the 40
transients. The data have been zero-filled twice to 1K in F1
dimension and weighted with shifted sine bell functions before
Fourier transform. A saturated solution of [NEt4]3a in CD2-
Cl2 was used for a 2D-NOESY experiment (Bruker DRX 300),
performed at 298 K in the phase-sensitive mode using a mixing
time of 2.2 s. There were 1K data points collected over a
spectral width of 783 Hz for each of the 16 transients. The
data have been zero-filled twice to 1K in F1 dimension and
weighted with shifted sine bell functions before Fourier
transform.
X-r a y An a lysis of 3a a n d 4. (a ) Collection a n d Red u c-
tion of X-r a y Da ta . Suitable crystals of 3a and 4 were
mounted in air on a glass fiber tip and then onto a goniometer
head. Table 2 contains the details of the crystal parameters
and data collection and refinement procedures. Single-crystal
X-ray diffraction data were collected on a Siemens SMART
CCD three-circle area detector at -50 °C using graphite-
monochromatized Mo KR radiation (λ ) 0.710 73 Å). Unit cell
parameters and an orientation matrix were obtained from
least-squares refinement on reflections measured in three
different sets of 15 frames each in the range 0° < θ < 23°.
The intensity data were collected using the ω-scan technique
within the limits 4° < 2 θ < 56°. Approximately a full sphere
of data was collected with the frame width set to 0.3°, the
sample-detector distance fixed at 5.5 cm, and a detector
exposition for each frame of 10 s. The first 100 frames were
recollected at the end of collection to monitor crystal decay,
which was not observed, thus no time-decay correction was
needed. The collected frames were processed by SAINT
software for integration; an absorption correction was applied
together with merging (SADABS).
Reaction of [P P h 4][Re3(µ-H)4(CO)9(Hpz)] ([P P h 4]6) with
P yr a zole a t 298 K. A sample of [PPh4]6 (11.6 mg, 0.0101
mmol) was dissolved in CD2Cl2 (0.5 mL) in a NMR tube and
treated with pyrazole (7.6 mg, 0.112 mmol). 1H NMR spectra,
acquired at different times, showed the clean transformation
of 6 into 3a and 3b, as illustrated in Figures 1 of the
Supporting Information. Due to the low concentration of 3b,
+
the large excess of free Hpz, and the presence of the PPh4
cation, only four of the pyrazolic resonances of 3b could be
detected at δ 10.6 (1, NH), 7.12 (2, HR,γ of pz), 6.14 (1, Hâ of
Hpz), and 5.80 (1, Hâ of pz) ppm. The same reaction was also
performed with D-pz, and NMR monitoring of the reaction
showed the appearance of a pseudotriplet, centered at δ 4.4
ppm, attributable to HD (J HD 43 Hz). Crystals of the [PPh4]+
salt of anion 3a suitable for X-ray analysis were grown by slow
diffusion of n-hexane into a concentrated CH2Cl2 solution. The
elemental analysis performed on some of these crystals gave
results in agreement with the presence of clathrated CH2Cl2,
revealed by the X-ray diffractometric investigation.
Rea ction of [NEt4]6 w ith CO. A sample of [NEt4]6 (8.0
mg, 0.0078 mmol) was dissolved in CH2Cl2 (ca. 8 mL), and the
solution was saturated with CO at room temperature. IR and
NMR analyses, after 2.5 h, showed the quantitative formation
of anion 1.
Rea ction of [NEt4]6 w ith P Me2P h . A sample of [NEt4]6
(12.3 mg, 0.012 mmol) in acetone-d6 was treated with PMe2-
Ph (1.5 µL, 0.0105 mmol) at 193 K. NMR spectra at 193 K
showed the slow formation of the substitution derivative
[Re3(µ-H)4(CO)9(PMe2Ph)]-, which became quantitative when
the temperature was raised to 298 K. NMR data of the
product (acetone-d6, 298 K): δ 7.4-7.7 (m, 5, Ph), 1.86 (d, 6,
Me, J PH 8.0 Hz), -7.57 (d, 1, J PH 6.1 Hz), -8.50 (d, 1, J PH 2.9
Hz), -12.58 (d, 2, J PH 17 Hz).
Rea ction of [P P h 4]6 w ith MeCN a t 298 K. A sample of
[PPh4]6 (11.3 mg, 0.0098 mmol) was dissolved in a NMR tube
in CD2Cl2 and treated with MeCN (7.4 µL, 0.129 mmol). The
sample was maintained at room temperature, and NMR
spectra were acquired at different times, showing mainly the
slow formation of the substitution derivative [Re3(µ-H)4(CO)9-
(NCMe)]- (8) and of the novel species [Re3(µ-H)3(µ-η2-pz)(CO)9-
(NCMe)]- (7) (6 h ca. 80%, 17%, 2%, and 1% of 6, 8, 7, and 3a ,
respectively; 22 h 63%, 26%, 9%, and 2%; 48 h 42%, 34%, 22%,
and 3%; 110 h 17%, 33%, 43%, and 7%). NMR data of 7 (CD2-
Cl2, 298 K): δ 7.11 (d, 2, HR,γ, J HH 1.9 Hz), 5.75 (t, 1, Hâ, J HH
1.9 Hz), 1.78 (s, 3, MeCN), -10.79 (s, 2, Ha), -11.66 (s, 1, Hb)
ppm.
(b) Solu tion a n d Str u ctu r e Refin em en t. The structures
have been solved by direct methods (SIR9228) and difference
Fourier methods and subsequently refined by full-matrix least-
2
squares against Fo using all reflections and the program
SHELXL9729 on a Silicon Graphics Indigo computer. Scat-
tering factors for neutral atoms and anomalous dispersion
corrections were taken from the internal library of SHELXL97.
All non-hydrogen atoms were given anisotropic displacement
Rea ction of [NEt4][Re3(µ-H)4(CO)9(P Me2P h )] w ith Hp z
a t 298 K. A sample of [NEt4][Re3(µ-H)4(CO)9(PMe2Ph)] (15
mg, 0.014 mmol) was dissolved in CD2Cl2 in a NMR tube and
treated with pyrazole (9.6 mg, 0.14 mmol). NMR monitoring
showed the slow appearance of the hydridic resonances at-
tributable to the novel anions [Re3(µ-H)3(µ-η2-pz)
(CO)9(PMe2Ph)]- (about 40% after 24 h) (9a and 9b, 4:1 ratio).
After 6 days, the conversion was complete. 1H NMR data of
the main product 9a (CD2Cl2, 298 K): δ 7.04 (d, 2H, HR,γ, J HH
(27) Bodenhausen, G.; Kogler, H.; Ernst, R. R. J . Magn. Reson. 1984,
58, 370.
(28) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.;
Burla, M. C.; Polidori, G.; Camalli, M., J . Appl. Crystallogr. 1994, 27,
435.
(29) Sheldrick, G. M. SHELXL-97: program for structure refinement;
University of Go¨ttingen: Go¨ttingen, Germany, 1997.
1.85 Hz), 5.81 (t, 1H, Hâ, J HH 1.85 Hz), 1.50 (d, 6H, Me, J HP
9

3292 Organometallics, Vol. 17, No. 15, 1998
Beringhelli et al.
parameters. Hydrogen atoms were first observed in a differ-
ence Fourier map and then included in idealized positions and
refined riding on their parent atoms with isotropic displace-
ment parameters of 1.2 times that of the pertinent atom.
Hydride positions were confirmed on the basis of the other
ligands stereochemistry, and optimized coordinates were
determined by means of HYDEX30 with dRe-H ) 1.85 Å.
The CH2Cl2 clathrated molecules in 4 were found disordered
with two-half molecules on a 2-fold axis (Wyckoff position 4e).
The final conventional agreement indexes are listed in Table
2. Selected bond parameters are reported in Table 3.
Ack n ow led gm en t. The authors warmly thank Mr.
Pasquale Illiano for his accurate and skillful collabora-
tion in the acquisition of the NMR data.
Su p p or tin g In for m a tion Ava ila ble: Tables of X-ray
analysis data, final atomic coordinates, anisotropic displace-
ment parameters, full bond distances and angles, and least-
squares planes and a figure concerning the reaction of [Re3(µ-
H)4(CO)9(Hpz)]- (6) with Hpz (17 pages). Ordering information
is given on any current masthead page.
(30) Orpen, A. G. J . Chem. Soc., Dalton Trans. 1980, 2509.
OM9800849